Show LESS errors in console, more thoroughly

Refinement of PR #8736: also log LESS errors from the parsing stage to the console (not just the output stage). Also covers cases where the file doesn't exist, for both CSS & LESS.
@redmunds The different error cases are already distinguishable because the error objects is included in the log message -- though the message isn't super clear in the case of a `loadFile()` failure due to the way `$.get()` passes error objects back.

We'd actually need 3 different fail handlers to cover all the cases, if we split the handler, since there are 3 different places where reject() could be called.  How about just detecting the two types of error results in a single handler, like this:

```
        result.fail(function (error, textStatus, httpError) {
            if (error.readyState !== undefined) {
                // If first arg is a jQXHR object, the real error info is in the next two args
                console.error("[Extension] Unable to read stylesheet " + path + ":", textStatus, httpError);
            } else {
                console.error("[Extension] Unable to process stylesheet " + path, error);
            }
        });
```

That improves the clarity of the message shown for loading errors, yet still guarantees we'll always log something in every failure case that could arise.